This research is concerned with an in vivo and in vitro study of positive and negative cellmediated immune regulation by biochemically purified soluble factors from normal and tumor-bearing host macrophages. It is proposed to determine: (1)\the biochemistry of macrophage-derived immunoregulatory substances; (2)\the intercellular mechanisms involved in macrophage-mediated modulation; and (3)\whether an in vivo mechanistic correlate can be demonstrated. Normal and tumor-bearing host mice were used to assess whether alteration in interleukins (IL) contributed to tumor-bearing host immune hyporeactivity. This was tested by evaluating titratable macrophage (MO) factor regulation of Lyt subpopulations and examining further along the immune cascade sequence by determining levels of IL-2 and IL-3. Anti-Lyt sera-separated Lyt+ cells were examined for mixed lymphocyte reactivity in the presence of normal and tumor-bearing host MO supernatants. The Lyt 2+ cells are more responsive to the enhancing activity of MO supernatantsthan were Lyt 1+ cells. Both T-cell phenotypes responded to the inhibitory substances. When comparing normal versus tumor-bearing host MO supernatants, the tumor-bearing host MO supernatants contained a higher level of inhibitory activity. No interferon or IL-2 activity was detected. There were low levels of IL-1 activity; however, tumor-bearing host MO supernatants contained slightly higher activity than their normal host counterparts. Supernatants from 24-hour concanavalin A-induced normal and tumor-bearing host spleen cells were tested for IL-2 and IL-3 activity. In a 28-day tumor growth kinetic study, IL-2 activity dropped significantly between days 8 and 10 and was undetectable at day 28. IL-3 activity also decreased significantly between days 4 and 8, and 12% of its activity was detectable on day 28. Admixture of purified IL-2 and IL-3 induced normal host splenocytes to a higher level of blastogenesis; however, IL-3 further suppressed tumor-bearing host spleen cell proliferation. Taken together, the paralysis of tumor-bearing host T-cell responses may be due to qualitative and/or quantitative impairments of immune-amplifying factors.